Use of an InGaAs Radiation Thermometer To Verify the Accuracy of the NPL Blackbody Reference Sources from 156 °C to 600 °C

Abstract

A transfer standard radiation thermometer based upon an InGaAs detector was developed at the NPL in 2001. The purpose of this instrument was to provide a compact high‐resolution device, which could be used to maintain and disseminate a radiance temperature scale between 156 °C and 962 °C. The thermometer requires calibration at the ITS‐90 fixed points. For this purpose fixed‐point blackbody sources have been designed and constructed using high‐purity metals comprising In, Sn, Zn, Al and Ag. Using the results of the calibration, an interpolating equation based upon the Wien function is calculated which relates the thermometer output to the radiance temperature of the source. This paper describes the development of the InGaAs thermometer with results which illustrate its stability between successive calibrations. The InGaAs thermometer was used to verify the calibration of the NPL variable‐temperature water and caesium heat‐pipe blackbody sources, whose temperatures are normally derived from ITS‐90 calibrated SPRTs inserted into the rear of the sources. This was achieved by comparing the radiance temperature of these sources obtained using the calibrated InGaAs thermometer with the ITS‐90 contact thermometry at various test temperatures. Allowing for measurement uncertainties and the emissivity of the reference sources, differences within ±0.1°C have been obtained at a number of test temperatures over the range 156 °C and 600 °C.